Fire has always been a hazard to oil and gas operations, and it is therefore essential that equipment used in the exploration for, and production of, oil and gas maintain its functional integrity under fire conditions to avoid worsening or proliferation of the fire. Valves are included among the equipment that sometimes becomes subjected to fire conditions and must, therefore, meet the requirements of the "API Recommended Practice Fire Test for Valves" of the American Petroleum Institute, referenced API RP6F, Second Edition, December, 1980. In summary those requirements include, in the case of a 5000 psi rated valve, for example, maintaining a 3750 psi (258.6 bar) upstream pressure in a test flame of between 1400.degree.-1600.degree. F. (761.degree.-871.degree. C.), a 5000 psi (345 bar) upstream pressure after cooling, and the ability to operate the valve against full rated pressure after it has cooled. A minimal amount of through and external leakage is permitted by the API RP6F requirements. Although the API RP6F criteria have proved beneficial in the design and development of fire-resistant valves, some companies, notably those within the Shell Group, expressed the belief that the fire test defined in API RP6F does not reflect the true temperatures experienced during an oil or gas fire. Those companies have also been concerned over the leakage permitted in the API RP6F test. A more stringent fire test was therefore developed by the Shell Group for qualification of fire-resistant wellhead equipment, including valves. This more stringent fire test is referred to as the API RP6F-"Improved" test, and is meeting with wide acceptance among both manufacturers and users of wellhead equipment as an aid in designing and testing of such equipment to improve its fire-resistant characteristics. The API RP6F-"Improved" fire test includes, for example, subjecting the equipment to a higher flame temperature of 2000.degree. F. (1100.degree. C.) than specified in API RP6F, and eliminating the allowable through and external leakage under prescribed test conditions. The API RP6F-"Improved" fire test is intended to be more severe than the actual temperatures experienced in a typical well fire. For a discussion of some recent efforts in design of fire-resistant wellhead equipment, including valves, and a comparison of the principal fire test conditions for the API RP6F and API RP6F-"Improved" procedures, reference may be had to "Development of Fire-Resistant Wellhead and Christmas Tree Equipment," by R. Hartley, P. Hamer and R. van Dort, Offshore South East Asia 82 Conference, February, 1982, and "Fire Resistant Wellhead Equipment for Statfjord `B` Platform," by Allen Millmaker and Manfred Leiser, Paper OTC 4371, Offshore Technology Conference, May, 1982.
In a conventional through-conduit, non-rising stem gate valve, the valve body has a flow way through which the pipeline fluids pass and a chamber intersecting the flow way inside the valve body. A gate is disposed within the chamber for reciprocation across the flow way where, in the upper position, for example, a flow port in the gate registers with the flow way to permit line fluids to flow through the valve, and in the lower position the gate blocks the flow through the flow way. The gate is reciprocated within the valve chamber by mechanical means, such as a hand wheel mounted on a stem connected to the gate, causing the gate to move across the flow way to open and close the valve. The gate is reciprocated on and sealed against a gate seat in the valve chamber, and the gate seat is mounted in and sealed with respect to the valve body. A bonnet closes the stem opening of the valve chamber. Stem packing seals between the stem and bonnet.
Under fire conditions, such a conventional gate valve may be engulfed in flames which may heat the valve to temperatures approaching 2000.degree. F. (1100.degree. C.). Such extreme heat causes the valve seals, including the stem packing, to deteriorate, and also causes the valve fluids to vaporize. The loss of seals causes external and through leakage. Vaporization of trapped valve fluids creates dangerously high pressures within the valve chamber, which can sometimes accelerate the loss of the deteriorating seals.
Fusible elements have been used in the past for safety valves and the like which are designed to remain open under normal environmental conditions, permitting flow of fluids through the valve, and to close under fire conditions, shutting off such flow.
U.S. Pat. No. 3,842,853 to Kelly et al., for example, discloses a heat responsive safety valve in one embodiment of which the gate is held in open position by an end cap secured to a nipple around the upper end of the stem by a fusible bushing, preventing upward movement of the stem. When the fusible bushing melts, it no longer holds the cap against the stem, and the stem moves upward responsive to a spring load to close the valve. In another embodiment, an outer sleeve is threaded to the nipple, and an inner cap is telescoped within the outer sleeve and prevented from rising with respect thereto by locking balls disposed in ports in the inner cap and a recess in the outer sleeve. A stem holding member is disposed on top of the stem, and a fusible disc is disposed between the upper end of the holding member and the inner cap. The holding member, restrained by the fusible disc, prevents the locking balls from moving inward. Upward movement of the holding member, inner cap and stem is thereby prevented during normal operations, thus holding the gate in open position. When the fusible disc melts, as in a fire, the fusible material is vented through a port in the inner cap and the holding member is allowed to rise. An annular recess in the holding member is allowed to register with the locking balls, permitting them to cam inwardly, releasing the inner cap from the outer sleeve and permitting the holding member, inner cap and stem to rise, closing the valve. In another embodiment, a cap having an internal annular recess is threaded onto the nipple, and a ring of fusible material is disposed in the radially outward end of the recess. A disc carrying a plurality of locking dogs is disposed above the top end of the stem, with the dogs being biased outwardly, partially into the recess and against the fusible ring, by upward stem force against the disc. Under normal conditions, the locking dogs prevent upward movement of the disc, thus holding the stem down and maintaining the valve in open position. In a fire, the fusible ring melts, venting the fusible material through ports in the cap, allowing the dogs to be cammed outwardly completely into the recess in the cap, thereby freeing the disc for upward movement and ejecting it from the cap. The stem is then permitted to rise, closing the valve. In the Kelly device, however, there is no provision for back-up seals in the event the O-ring stem seal deteriorates in the fire, and thus external leakage past the O-ring stem seal may be permitted even though the valve is closed. This is unacceptable in a fire-resistant valve. Moreover, in the Kelly device, the elements holding the stem down under normal operations are free to be forcefully ejected from the valve during a fire, which is extremely hazardous to other equipment and personnel in the vicinity. This is also unacceptable.
Fusible elements have also been used in valves in the past which, under normal environmental conditions, remain integral to allow routine operations of the valve, and under fire conditions become fused to permit creation of a secondary, fire-resistant, typically metal-to-metal seal to prevent loss of fluid through a deteriorating or destroying primary seal.
U.S. Pat. No. 2,647,721 to Volpin discloses a fusible ring around a valve stem between a packing gland and bearing arranged to melt under elevated temperatures to permit the bearing and the stem to move axially upward so that an annular shoulder on the stem engages a corresponding seat in the valve body forming a metal-to-metal seal.
U.S. Pat. No. 3,788,600 to Allen discloses a gate valve with a two-piece stem, the outer section of which telescopes with respect to the inner section. The outer section has a metal sealing shoulder which is held apart from a metal seat in the bonnet cap by a fusible ring. In the event of a fire, the fusible ring melts, allowing the upper stem section to move upwardly so that its sealing shoulder engages the seat in the bonnet cap, forming a metal-to-metal seal.
U.S. Pat. No. 4,082,105 to Allen discloses a valve having annular elements which are held in a distorted position (see FIG. 2 of Allen '105) by means of solder. When the valve is subjected to sufficient heat, the solder melts and the annular elements relax to a position such that their inner edges bite into the valve stem to form a secondary seal, as shown in FIG. 3 of Allen '105.
U.S. Pat. No. 4,214,600 to Williams, Jr. et al. discloses a fusible washer in a valve bonnet cap between an upper stem bearing and the inside end of the cap which during normal operations supports the bearing and valve stem such that a shoulder on the stem is spaced from a corresponding backseat in the bonnet. When the fusible washer is exposed to fire, it melts and is drained outside the valve body. As the fusible washer melts, it no longer supports the bearing and valve stem, permitting the stem shoulder to move into metal-to-metal sealing engagement with the bonnet backseat.
U.S. Pat. No. 4,245,661 to McGee discloses a heat responsive backseat arrangement for a valve stem. An upper stem bearing is supported above by a fusible annular disc with outlet ports provided to allow the fusible material to escape when melted. When the fusible annular disc melts, as in a fire, the valve stem and its bearings are permitted to rise relative to the bonnet, permitting a shoulder on the stem to engage a backseat in the bonnet to form a metal-to-metal seal.
U.S. Pat. No. 4,271,857 to Rowe discloses a valve actuator on a valve bonnet, including a piston stop sleeve supported in a normal position by a fusible ring which provides an upper travel stop for the actuator piston and stem under normal operating conditions, preventing a shoulder on the stem from backseating in the bonnet. When the fusible ring melts, as during a fire, the fusible material is vented through a port and the stop sleeve is allowed to rise, enabling the actuator piston and stem to rise beyond the normal stop position and the stem shoulder to backseat, forming a metal-to-metal seal.
See also, the two above-referenced technical papers wherein there is discussion concerning use of a Cameron gate valve, modified to include a spacer ring of eutectic material beneath the stem bearing which allows metal-to-metal backseating of the stem under fire conditions.
Several problems exist with the prior art devices discussed above that sometimes lead to inadequate performance of the equipment under fire conditions. In addition to the drawbacks with the approach taken in the Kelly patent, discussed supra, the relatively large volume of fusible material utilized in the prior art fusible elements leads to a slower reaction time of the fusible elements during a fire, when rapid response of the devices is extremely critical to ward off worsening or proliferation of the fire. Moreover, the fusible elements are located in a position such that heat conduction to the fusible elements is relatively poor, further slowing reaction time. In addition, the relatively large volume of fusible material utilized in the fusible elements often necessitates providing special relief holes in the valve for venting the fused material. In the event such a relief hole were to become plugged, the device may not operate or, if it does, it may not do so quickly enough. The fusible material may re-solidify in the bearings, rendering the valve inoperable. Further, especially with the devices designed to create a secondary metal-to-metal backup seal for a deteriorating or destroyed primary seal, it is not readily apparent to an observer whether or not the fusible material has been melted by the fire, and thus whether refitting of the bonnet is necessary.
It is an object of the present invention to provide a fire responsive stem retention apparatus utilizing fusible materials for a fire resistant, non-rising stem valve which under normal environmental conditions allows routine operation of the valve with no backseating of the valve stem, and under fire conditions permits backseating of the valve stem in the bonnet, creating a secondary, metal-to-metal or other fire resistant seal, as of compacted graphite, as a backup for the primary stem seal, and which overcomes the above-described problems associated with prior art devices. The fire responsive stem retention apparatus of the present invention is designed for reliable, more rapid response to fire conditions through the use of a smaller volume of fusible material than in prior art devices, and more intimate contact between the fusible material and the portions of the valve exposed to the fire, resulting in improved heat conduction to the fusible material. In using a smaller volume of fusible material, the apparatus of the present invention does not require special relief holes to vent the fusible material when melted, nor can the fusible material foul the bearing or solder the stem to the bonnet. The apparatus of the present invention also provides a ready external visual indication of the status of the fusible material, and, therefore, of whether or not refitting of the bonnet is necessary. It is also an object of the present invention to provide such a fire responsive stem retention apparatus for an otherwise fire resistant valve enabling it to successfully pass the fire tests set out in the API RP6F and API RP6F-"Improved" procedures.
Other objects and advantages of the present invention will become apparent from the following detailed description.